1. Field of the Invention
This invention relates to a method for the production of an alkanolamine by the reaction of ammonia and an alkylene oxide using a solid catalyst, and an apparatus used therefor.
2. Description of Related Art
A method for producing ethanolamine by causing ethylene oxide (C2H4O) to react with aqueous ammonia (NH3) solution (with the ammonia concentration in range of 20 to 40% by weight) has been adopted as a means to effect commercial production of alkanolamines. The ethanolamine thus produced is in the form of a mixture of three components, monoethanolamine (NH2CH2CH2xe2x80x94OH), diethanolamine [NH(CH2CH2xe2x80x94OH)2], and triethanolamine [N(CH2CH2xe2x80x94OH)3]. The method represses the formation of triethanolamine. The reaction, therefore, is generally carried out with the molar ratio of ammonia to ethylene oxide set at a large ammonia excess in the approximate range of 3 to 5:1. In spite of the effort, the selectivity for triethanolamine is in the range of 10 to 20% by weight or more and the selectivity for diethanolamine is not more than 40% by weight.
This method for producing an alkanolamine using aqueous ammonia solution may be referred to as xe2x80x9caqueous ammonia solution method for production of alkanolamine.xe2x80x9d
Substantially no reaction occurs between the alkylene oxide and ammonia in the system using no aqueous ammonia solution, namely anhydrous system. For this reaction, therefore, a catalyst, such as an ion-exchange resin, molecular sieve, silicia-alumina, acid-activated clay, a catalyst in which a rare earth element is supported on heat-resistant carriers, is indispensable.
U.S. Pat. No. 6,169,207 B1 discloses a catalyst which inhibits the formation of trialkanolamine and permits selective production of dialkanolamine. European Patent Publication Nos. 104363 and 1104752 disclose methods for starting up reactions, methods for packing catalysts, reaction apparatuses, and methods for regenerating catalysts. No mention is made anywhere in these references about a process including a system for recovering unreacted ammonia.
In the production of an alkanolamine by the reaction of liquid ammonia and an alkylene oxide, recovery of unreacted ammonia is required since the reaction uses excess ammonia. In order for this unreacted ammonia to be recovered as liquid ammonia, an ammonia-recovering column must be pressurized. During the application of pressure, the ammonia concentration in the bottom product of the column is increased to a level of about 4 to 20% by weight. From the bottom product, therefore, ammonia cannot be recovered through just one stage of distillation. It is further necessary that the temperature of the bottom product of the column, be held down as low as possible to prevent the alkanolamine from being degraded. The bottom product temperature, for example, must be held down to not more than 300xc2x0 C., preferably not more than 250xc2x0 C. The removal of ammonia from the bottom product under such conditions entails decompressing the column. To recover ammonia at the top of the column, it is necessary to lower the temperature to a very low level by the use of a refrigerator or transform the ammonia into liquid ammnonia by compression with a compressor. This entails problems such as increasing the size of the equipment, increasing the construction cost, and incurring much expense.
Meanwhile, when improving the production of alkanolamine, in particular the dialkanolamine, and when a catalyst having high selectivity to dialkanolamine is used, the product contains mainly mono alkanolamines. The selectivity to dialkanolamines is not sufficiently high unless the concentration of alkylene oxides is increased. Incidentally, when the concentration of alkylene oxides is excessively high the reaction heat per unit of the product solution will be inevitably large and the adiabatic reaction will not occur because of an undue rise of temperature. For solving this problem, multistage adiabatic reactions that use a plurality of reactors and that supply divided alkylene oxide may be carried out. This reaction uses liquid ammonia as a raw material at an elevated temperature. Therefore, the reaction needs a plurality of reactors capable of withstanding very high pressure and, as a result, raises the cost of equipment.
An object of this invention is to solve such problems, and to provide a method for recovering unreacted ammonia using water as an absorbent in the production of alkanolamines without lowering the temperature of the bottom product with a refrigerator or lowering the pressure thereof with a compressor.
According to the first aspect of this invention, it provides a method for producing an alkanolamine by the reaction of liquid ammonia and a corresponding alkylene oxide using a solid catalyst, which method comprises the following steps:
a) recovering not less than 60% by weight of the ammnonia, based on the weight of a product solution which is emerged from the outlet of a reactor and contains unreacted ammonia and alkanolamines, as liquid ammonia and the remainder thereof as a bottom product at an ammonia-recovering column;
b) directing the bottom product to an ammonia-stripping column in which the ammonia is stripped out; and
c) feading the stripped ammonia to an ammonia absorption column in which the ammonia is absorbed by means of water, thereby recovering it as aqueous ammonia solution.
According to the present invention, it can produce alkanolamines by the reaction of liquid ammonia and the corresponding alkylene oxide inexpensively and efficiently without expensive devices such as refrigerators and compressors, which are used in the ammonia removal and ammonia recovery from the product solution containing ammonia, e.g., 4 to 20% by weight of ammonia.
Further, we have conducted a diligent study with a view to solving the problems mentioned above, and, as a result, found that by circulating part of the product solution formed in the reactor to the inlet to the reactor, it is possible to obtain an alkanolamine, in particular selectively dialkanolamine, of a required composition by the use of an adiabatic reactor. This invention has been perfected as a result.
According to the second aspect of this invention, it can provide a method for producing an alkanolamine by the reaction of liquid ammonia and a corresponding alkylene oxide effected by the use of a fixed-bed reactor in the presence of a solid catalyst, which method is characterized by having part of the product solution sent to the reactor.
Then, according to the third aspect of this invention, it can provide an apparatus to be used in the production of an alkanolamine by the reaction of liquid ammonia and a corresponding alkylene oxide in the presence of a solid catalyst, which apparatus is characterized by comprising a fixed-bed reactor, a pressure-controlling valve disposed on the downstream side of the reactor, a pump for circulating the product solution to the reactor, a booster pump for supplying the raw material ammonia to the reactor, and a booster pump for supplying the raw material alkylene oxide to the reactor and consequently allowing control of the amount of the product solution to be circulated to the reactor by means of the pump mentioned above.
According to this invention, it is possible to carry out the reaction of an alkylene oxide and liquid ammonia by the use of one reactor and produce a dialkanolamine selectively and efficiently, and it is further possible to provide an apparatus for carrying out this reaction.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.